1. Field of the Invention
The present invention relates to a communication system wherein a plurality of network devices and a plurality of relay servers are connected by a network, and the relay server preferable to be used in such communication system.
2. Description of the Related Art
FIG. 11 is a view useful for explaining an example of a system wherein a general Internet is used. In the figure, the reference numbers 1, 2 are local systems, 3 is Internet, 11, 12, 21, 22 are terminals, 13, 23 are gateways, and 14, 24 are LAN. The local system 1 is constructed by the terminal 11, the terminal 12, the gateway 13 or the like being connected by the LAN 14. The gateway 13 is connected to the Internet 3 along with the LAN 14, and one can use the Internet from various network apparatus such as the terminal 11, the terminal 12 or the like on the LAN 14. Moreover, the local system 2 is also constructed in the same manner such that it is constructed by the terminal 21, the terminal 22, the gateway 23 or the like being connected by the LAN 24. The gateway 23 is connected to the Internet 3 along with the LAN 24, and one can use the Internet from various network apparatus such as the terminal 21, the terminal 22 or the like on the LAN 24. Moreover, in each of the local system 1 and the local system 2, other various apparatuses can be connected by the LAN 14 and the LAN 24.
In such a system, generally, one or a plurality of global IP addresses are assigned to the local system 1 and the local system 2, but the global IP address is not necessarily assigned to each of the network apparatuses within the local system 1 and the local system 2. A private IP address is assigned to each network apparatus within each of the local system 1 and the local system 2, and the private IP address is converted into the global IP address by using functions such as NAT or IP masquerade by the gateway 13 and the gateway 23. By using the gateway 13 and the gateway 23 including such an IP address converting function, for example in the local system 1, the terminal 11 and the terminal 12 are to use the Internet 3 via the gateway 13. Moreover, in the local system 2, the terminal 21 and the terminal 22 are to use the Internet 3 via the gateway 23.
Moreover, the gateway 13, the gateway 23 or other network devices or the like is provided with a function such as a fire wall or a proxy server, and a structure in which the respective terminals use the Internet 3 via these devices is used, and the safety in the system is improved.
For example, when one attempts to access the terminal 11 within the local system 1 from the Internet 3, the global IP address of the gateway 13 can be learned, but the private IP address of the terminal 11 cannot be learned. Therefore, in the general connecting method, the terminal 11 cannot be accessed from outside the local system 1. Moreover, there are cases in which by the function of the fire wall of the gateway 13 for example, the site for accepting the access is limited. Further, it is to be the same manner in the terminal 12, and also in the terminal 21 and the terminal 22 within the local system 2.
Furthermore, the terminal 11 or the terminal 12 within the local system 1, or the terminal 21 or the terminal 22 within the local system 2 is generally provided only with a client function, and is not provided with a function of a server for receiving information from a different network apparatus. Therefore, unless accessing from the terminals 11, 12, 21, 22, to the different network apparatus, the information cannot be transmitted to these terminals from the different network apparatus.